Definition 2.12 (cf. [1]}) 
Let \(f : X \rightarrow \overline{\mathbf {R}}\)  and \(x,v \in X\) . The one-sided
directional derivative of \(f\)  at \(x\)  with respect to \(v\) is defined to be
\(f^{\prime }(x;v) = \lim _{\lambda \rightarrow 0+}\frac{f(x+\lambda v) - f(x)}{\lambda } \,,\) 
